1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching mode liquid crystal display device, the storage capacitance of which is increased so as to minimize the aperture ratio due to the area of a storage capacitor electrode.
2. Discussion of the Related Art
A liquid crystal display device, which is one of widely used flat panel display devices, is a device including a liquid crystal having the fluidity of a liquid and the optical properties of a crystal wherein an electric field is applied to the liquid crystal to change optical anisotropy of the liquid crystal. The power consumption of the liquid crystal display device is lower than that of a conventional cathode ray tube, and the size of the liquid crystal display device is less than that of the conventional cathode ray tube. Furthermore, the liquid crystal display device can be manufactured in large size and high definition. Consequently, the liquid crystal display device is widely used.
Based on the properties of a liquid crystal and the structure of a pattern, liquid crystal display devices may be constructed in various different modes.
Specifically, liquid crystal display devices are classified into a twisted nematic (TN) mode liquid crystal display device, a multi-domain mode liquid crystal display device, an optically compensated birefringence (OCB) mode liquid crystal display device, an in-plane switching mode liquid crystal display device, and a vertical alignment mode liquid crystal display device. In a twisted nematic (TN) mode liquid crystal display device, liquid crystal directors are arranged such that the liquid crystal directors are twisted 90 degrees, and a voltage is applied to the liquid crystal directors to control the liquid crystal directors. In a multi-domain mode liquid crystal display device, a pixel is divided into several domains, and directions of main viewing angles of the respective domains are changed to accomplish a wide viewing angle. In an optically compensated birefringence (OCB) mode liquid crystal display device, a compensating film is attached to the outer circumferential surface of a substrate to compensate for the phase change of light depending upon the path of the light. In an in-plane switching mode liquid crystal display device, two electrodes are formed on a substrate such that liquid crystal directors are twisted on the even plane of an alignment film. In a vertical alignment mode liquid crystal display device, a negative liquid crystal and a vertical alignment film are disposed such that the major axis of a liquid crystal molecule is aligned vertically to the plane of the alignment film.
The in-plane switching mode liquid crystal display device generally comprises a color filter array substrate and a thin film transistor array substrate, which are opposite to each other. Between the color filter array substrate and the thin film transistor array substrate is disposed a liquid crystal layer.
On the color filter array substrate are formed a black matrix for preventing light leakage and an RGB color filter layer for realizing colors on the black matrix.
On the thin film transistor array substrate, are formed gate lines and data lines, by which each pixel unit is defined, a switching element at the intersection between the gate lines and the date lines, and common electrodes and pixel electrodes, which are alternately arranged to generate a transversal electric field.
A conventional in-plane switching mode liquid crystal display device will be described with reference to the accompanying drawings.
FIG. 1 is a plan view illustrating a pixel unit of the conventional in-plane switching mode liquid crystal display device, and FIG. 2 is a cross-sectional view taken along line I-I′ of FIG. 1.
As shown in FIGS. 1 and 2, the conventional in-plane switching mode liquid crystal display device includes a thin film transistor array substrate having a plurality of gate lines 12 arranged in line and a plurality of data lines 15 intersecting at right angles to the gate lines 12. Each pixel unit is defined by the gate line 12 and the data line 15. At each pixel unit are disposed a thin film transistor (TFT) serving as a switching element, a plurality of common electrodes 24 arranged parallel with the data lines 15, and a plurality of pixel electrodes 17 arranged parallel with the common electrodes 24. The pixel electrodes 17 are disposed between the common electrodes 24 such that the pixel electrodes 17 and the common electrodes 24 are alternately arranged.
The common electrodes 24 are connected to a common line 25 via a second contact hole 82 such that a Vcom signal is transmitted to the common electrodes 24 from an external drive circuit.
One end of each of the pixel electrodes 17 is integrally connected to each other. The integrally connected ends of the pixel electrodes 17 are connected to a drain electrode 15b of the thin film transistor (TFT) via a first contact hole 81 such that a pixel signal is transmitted to the pixel electrodes 17.
The common lines 25 and the gate lines 12 are made of a opaque low-resistance metal material. The common lines 25 and the gate lines 12 are formed on the same layer of the substrate. The data lines 15 are formed above the common lines 25 and the gate lines 12 while a gate insulation film 13 is disposed between the data lines 15 and the gate lines 12. The pixel electrodes 17 and the common electrodes 24 are formed on the same layer above the data lines 15 while a protective film 16 is disposed between the pixel electrodes 17 and the data lines 15. The pixel electrodes 17 and the common electrodes 24 are made of a transparent conductive material, such as indium tin oxide (ITO). The structure in which the pixel electrodes 17 and the common electrodes 24 are formed of the transparent conductive material, such as ITO, is referred to as an ITO-ITO electrode structure.
In this case, a storage capacitor is further provided to maintain electric charge applied in the liquid crystal at the turnoff section of the thin film transistor to prevent deterioration of the image quality due to the parasitic capacitor. In the case of the ITO-ITO structured in-plane switching mode liquid crystal display device, as shown in FIG. 2, a predetermined portion of the common line 25 serves as a lower capacitor electrode 90, and predetermined portions of the pixel electrodes 17 overlapping with on the lower capacitor electrode 90 serve as an upper capacitor electrode 91. The gate insulation film 13 and the protective film 16 disposed between the common line 25 and the pixel electrode 17 serve as an insulation film between the capacitor electrodes. In this way, a storage capacitance Cst is formed. The lower capacitor electrode is formed of an opaque layer. Therefore, the aperture ratio is decreased due to the large area of the opaque layer.
Recently, a larger number of pixel sections have been formed at the in-plane switching mode liquid crystal display device although the size of the liquid crystal panel is fixed. Consequently, the resolution of the thin film transistor liquid crystal display device has increased, and therefore, the size of a pixel unit section has decreased. However, the area of the capacitor electrode for forming the storage capacitor is not reduced. As a result, the aperture ratio of the pixel unit section is decreased due to the relatively large opaque capacitor electrode.